Restoration of alkali hydroxide etchants of aluminum

Abstract

An alkali hydroxide solution of a metal hydroxide or metallic oxide acidic is separated into a solid metal hydroxide or metallic oxide acidic and a solution of alkali hydroxide by feeding the alkali solution to an electrolyte comprising a soluble anion of an acid and controlling the pH of the electrolyte to insolubilize a metal hydroxide or metallic oxide acidic by electrotransporting alkali cations from the electrolyte through a cation permeable membrane into a catholyte and converting the alkali cations to a solution of alkali hydroxide. Sodium hydroxide etchants of aluminum containing sodium aluminate are continuously converted to aluminum hydroxide and a solution of sodium hydroxide.

I claim: 
     
       1. A method for insolubilizing a metal hydroxide in a solution of an alkali hydroxide, comprising the steps of feeding alkali hydroxide solution heavy metal hydroxide to an anolyte compartment of an electrodialysis cell containing an electrolyte having a soluble anion of an acid, and applying electrical energy to the cell to electrotransport alkali cations from the said electrolyte to a catholyte compartment of the cell containing an alkali hydroxide catholyte through a cation permeable membrane separating said compartments, at a rate to control the pH of said electrolyte and to cause said heavy metal hydroxide to separate as a solid in said electrolyte, and to produce an alkali hydroxide in the catholyte. 
     
     
       2. The method of claim 1 wherein said soluble anion of an acid is selected from the group consisting of acids of sulfur, halogen, nitrogen, phosphorous, and carbon, which acids in a one normal solution have a pH no greater than three (3). 
     
     
       3. The method of claim 1 wherein the alkali hydroxide is a hydroxide of an alkali metal from the group of alkali metals consisting of sodium and potassium. 
     
     
       4. The method of claim 1 wherein said metal hydroxide is selected from the group of hydroxides consisting of aluminum, lead, tin, zinc, and gallium. 
     
     
       5. The method of claim 1 wherein said soluble anion of an acid is selected from the group consisting of acids of sulfur, halogen, nitrogen, phosphorous, and carbon, which acids in a one normal solution have a pH no greater than one (1). 
     
     
       6. The method of claim 1 wherein the alkali hydroxide is a hydroxide of ammonium from the group consisting of sodium and potassium. 
     
     
       7. A process using an electrodialysis cell for the insolubilization of a metal hydroxide from a solution of an alkali hydroxide by feeding it as a first aqueous solution to an anolyte compartment containing an anolyte having a soluble anion of an acid, passing an electric current through said electrodialysis cell to electrotransport alkali cations from said first aqueous solution in the anolyte compartment of said electrodialysis cell through a cation permeable membrane into a catholyte compartment of said electrodialysis cell which contains a second aqueous solution of a weak alkaline hydroxide at a rate to control the pH of said first aqueous solution to cause said metal hydroxide to separate as a solid in said first aqueous solution, and to produce in said second aqueous solution an alkali hydroxide such solution being in electrical communication with the cathode of said electrodialysis cell. 
     
     
       8. The process of claim 7 wherein said soluble anion of an acid is selected from the group consisting of acids of sulphur, halogen, nitrogen, phosphorous and carbon which acids in a one normal solution would have a pH no greater than three (3). 
     
     
       9. The process of claim 7 wherein said alkali hydroxide is a hydroxide of an alkali metal from the group consisting of sodium and potassium. 
     
     
       10. The process of claim 7 wherein the metal hydroxide is selected from the group consisting of hydroxides of aluminum, lead, tin, zinc, and gallium. 
     
     
       11. A process using an electrodialysis cell for the insolubilization of aluminum hydroxide dissolved in a solution of sodium hydroxide comprising the steps of feeding to a first aqueous anolyte solution contained in the anolyte compartment of the cell, such anolyte having a soluble anion of an acid, passing an electric current through said electrodialysis cell to electrotransport sodium cations from said first aqueous solution through a cation permeable membrane into a catholyte compartment containing a second aqueous solution at a rate to control the pH of said first aqueous solution to cause said dissolved aluminum hydroxide to separate as a solid in said first aqueous solution and to generate sodium hydroxide said second aqueous solution in said second compartment where such solution is in electrical communication with the cathode of said electrodialysis cell. 
     
     
       12. The process of claim 11 wherein said soluble anion of an acid is selected from the group consisting of acids of sulfur, halogen, nitrogen, phosphorous and carbon, the acids of which in a one normal solution have a pH no greater than three (3). 
     
     
       13. The process of claim 12, where the soluble anion is a sulfate anion. 
     
     
       14. The process of claim 13 wherein said second aqueous solution is a solution of an alkali hydroxide in electrical contact with a cathode of said electrodialysis cell. 
     
     
       15. The process of claim 11 wherein the pH of said anolyte is greater than five and less than fourteen. 
     
     
       16. The process of claim 11 wherein said second aqueous solution is a solution of sodium hydroxide in electrical contact with a cathode of said electrodialysis cell. 
     
     
       17. A method for insolubilizing a metallic oxide acidic in a solution of an alkali hydroxide, comprising the steps of feeding said solution to an anolyte compartment of an electrodialysis cell containing an electrolyte having a soluble anion of an acid and applying electrical energy to the cell to electrotransport alkali cations from the said electrolyte to a catholyte compartment of the cell containing an alkali hydroxide catholyte, through a cation permeable membrane separating said compartments, at a rate to control the pH of said electrolyte and to cause said metal hydroxide to separate as a solid in said electrolyte, and to produce an alkali hydroxide in the catholyte. 
     
     
       18. A process using an electrodialysis cell for the insolubilization of a metallic oxide acidic from a solution of an alkali hydroxide by feeding it as a first aqueous solution to an anolyte compartment containing an anolyte having a soluble anion of an acid, passing an electric current through said electrodialysis cell to electrotransport alkali cations from said first aqueous solution in the anolyte compartment of said electrodialysis cell through a cation permeable membrane into a catholyte compartment of said electrodialysis cell which contains a second aqueous solution of a weak alkaline hydroxide, at a rate to control the pH of said first aqueous solution to cause said metal hydroxide to separate as a solid in said first aqueous solution, and to produce in said second aqueous solution an alkali hydroxide such solution being in electrical communication with the cathode of said electrodialysis cell. 
     
     
       19. A method for insolubilizing a metallic oxide acidic in a solution of an alkali hydroxide, comprising the steps of feeding the said solution to an anolyte compartment of an electrodialysis cell containing an electrolyte having a soluble anion of an acid and applying electrical energy to the cell to electrotransport alkali cations from the said electrolyte to a catholyte compartment of the cell containing an alkali hydroxide catholyte, through a cation permeable membrane separating said compartments, at a rate to control the pH of said electrolyte and to cause said metal hydroxide to separate as a solid in said electrolyte, and to produce an alkali hydroxide in the catholyte, wherein said metallic oxide acidic is selected from the group consisting of acidic oxides of tungsten, molybdenum, vanadium, niobium, tantalum and uranium. 
     
     
       20. A process using an electrodialysis cell for the insolubilization of a metal hydroxide from a solution of an alkali hydroxide comprising the steps of: feeding said solution as a first aqueous solution to an anolyte compartment which contains an anolyte having a soluble anion of an acid, passing an electric current through said electrodialysis cell to electrotransport alkali cations from said first aqueous solution in the anolyte compartment of said electrodialysis cell through a cation permeable membrane into a catholyte compartment of said electrodialysis cell which contains a second aqueous solution of a weak alkaline hydroxide at a rate to control the pH of said first aqueous solution and to cause said metal hydroxide to separate as a solid in said first aqueous solution, and to produce in said second aqueous solution an alkali hydroxide, such solution being in contact with the cathode of said electrodialysis cell, and wherein the metal hydroxide is selected from the group consisting of hydroxides of aluminum, lead, tin, zinc, and gallium. 
     
     
       21. A process using an electrodialysis cell for the insolubilization of a metal oxide acidic from a solution of an alkali hydroxide comprising the step of feeding said solution as a first aqueous solution to an anolyte compartment which contains an anolyte having a soluble anion of an acid, passing an electric current through said electrodialysis cell to electrotransport alkali cations from said first aqueous solution in the anolyte compartment of said electrodialysis cell through a cation permeable membrane into a catholyte compartment of said electrodialysis cell which contains a second aqueous solution of a weak alkaline hydroxide at a rate to control the pH of said first aqueous solution to cause said metallic oxide acidic to separate as a solid in said first aqueous solution, and to produce in said second aqueous solution an alkali hydroxide, such solution being in contact with the cathode of said electrodialysis cell, and wherein the metal hydroxide is selected from the group consisting of hydroxides of aluminum, lead, tin, zinc, and gallium.